The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is ...located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.
We used an InSb radiometric thermal imager to characterize the performance of 1'' X 1'' negative luminescent (NL) arrays. The devices grown on both CdZnTe (two arrays) and silicon (three arrays) as ...substrates have cut-off wavelengths ranging from 5.3 mum to 6.0 mum. The reverse-bias saturation current densities range from 0.3 A cm (lambda co = 5.3 mum) to 1 A cm (lambda co = 6.0 mum). The apparent array temperatures decrease by 37.9 K to 42.8 K under reverse bias, which corresponds to external NL efficiencies of 80-85%. Most of the inefficiency results from the non-ideal AR coating, whose reflectivity is approx 15% when weighted over the black body and atmospheric transmission spectra. It is highly encouraging that both the electrical and NL properties are slightly superior for the devices grown on silicon substrates.
We demonstrate the substantial suppression of infrared (IR) blackbody emission from HgCdTe photodiode arrays with cutoff wavelengths of 4.8, 5.5, and 6.0 mu m. At room temperature, a reverse bias ...induces internal negative luminescence (NL) efficiencies of 95%, 93%, and 88%, respectively, which correspond to apparent cooling of the surface temperatures by 60, 59, and 49 K. Reverse-bias saturation current densities for the three devices were 0.11, 0.7, and 32 A/cm super(2). Measurement and analysis of the material transmission characteristics indicate that the small residual inefficiencies may be limited by a parasitic absorption process. The 4.8 and 5.5 mu m photodiodes were fabricated into 18 2 arrays with total areas of 5 mm 5 mm. In both cases, all 36 array elements were fully operable, and had similar electrical and NL properties. Effective fill factors were approximately 100%, since carrier diffusion led to the extraction of carriers from regions between the elements. These results show that efficient, low-power NL devices with active areas in the square-inch range are now feasible for such applications as the cold shielding of infrared focal-plane arrays.
Negative luminescence (NL) refers to the suppression of infrared blackbody emission, and hence an apparent temperature reduction, due to free carrier extraction from a reverse-biased p-n junction. A ...number of applications are envisioned for NL devices, including cold shielding of background-limited uncooled and cryogenic focal-plane arrays, dynamic nonuniformity correction for ir imaging, and ir scene simulation. High-performance NL devices have recently been demonstrated. For example, a HgCdTe/CdZnTe photodiode with 4.8-µm cutoff wavelength achieved an internal NL efficiency of 95% at room temperature. This means that the blackbody emission was suppressed by a factor of 20 and that the apparent temperature of the device surface decreased by 60 K. The corresponding reverse-bias saturation current density was 0.11 A/cm^sup 2^. Even HgCdTe devices (λ^sub co^ = 5.3 µm) grown on large-area silicon substrates with substantial lattice mismatch displayed 88% internal NL efficiency and saturation current densities no larger than 1.3 A/cm^sup 2^. These results indicate a clear path toward a negative-luminescence device technology that is efficient, operates at low power, and is inexpensive. PUBLICATION ABSTRACT
•We assessed rates of traditional and cyber victimization among a college-aged sample.•We examined associations among victimization, social support, and depression.•Traditional and cyber ...victimization were positively associated with depression.•Cybervictimization better predicted depression than did traditional victimization.•Social support was negatively associated with symptoms of depression.
Data on students’ perceptions of social support, traditional and cyber victimization behavior, and social-emotional well-being were collected from a sample of 267 university students in the Midwestern United States. One purpose of the current study was to examine possible sex differences in perceptions of the prevalence of cybervictimization experiences. The current study also examined whether cybervictimization accounted for additional variance in depression beyond traditional victimization and if social support would buffer the associations among traditional and cyber victimization and depression. Young men and women did not report significantly different rates of cybervictimization. Cybervictimization was significantly related to depression above and beyond that of traditional victimization. Social support was negatively related to depression. However, there was no moderating role of social support. The results of this study highlight the significant impact that traditional and cyber victimization may have on college students’ well-being. Social support was also an important factor in the relation to depression, however, it did not provide a buffer in the association between victimization and depression.
This paper analyses the electro-optical behavior of simple, near-optimal MWIR HgCdTe photodiodes. These devices operate near fundamental materials limits making them both excellent in quality and ...ideal for understanding the most basic aspects of infrared photodiode performance. Measurements of representative diodes are explained by models that are simple but still accurate in describing optical and electrical properties.
Over the last several years cooled applications of HgCdTe at low temperatures have proliferated. Having low fundamental dark current at any given wavelength and temperature makes HgCdTe attractive ...for high temperature applications as well. We are exploring detectors with cut off wavelengths from the near to middle infrared region (approximately 1.5 to approximately 4 mu m). Theory allows applications from low light level imaging in starlight and `nightglow' to thermal imaging, both with useful sensitivities at room temperature. The demonstrated possibility of reducing or eliminating traditional recombination processes (radiative and Auger) further increase the attractiveness of HgCdTe. Current materials technology shows some evidence that these sensitivities can be attained. Current detector technology, being limited by SRH traps, appears to require modest cooling (to about 250 K). Improved materials and processes should eliminate the need for even this cooling.
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